Method of processing a mixture of liquefied gases



y 1966 A. HARMENS 3,253,418

METHOD OF PROCESSING A MIXTURE 0F LIQUEFIED GASES Filed Nov. 19, 1964 Product Leon Gos Merhone 2 Mol.%

Erhone 58 Mol.% 9 Liquid Propane Propane 4O Mol.% W

Rich Gus Product INVENTOR A Iexonder Harmens ATTORNEY United States Patent Ofiiice 3,253,418 METHOD OF PROCESSING A MIXTURE F LIQUEFIED GASES Alexander Harmens, Purley, England, assignor to Conch International Methane Limited, Nassau, The Bahamas, a Bahamian company Filed Nov. 19, 1964, Ser. No. 412,383 Claims priority, application Great Britain, Feb. 11, 1964, 5,743/ 64 7 Claims. (Cl. 6252) This invention relates to a method of processing a mixture of liquefied gases. More particularly, it concerns a method of vaporizing liquefied natural gas in such a way as to obtain its components in an advantageous gaseous form and at the same time make best use of the liquefied natural gas as a sink of heat, namely to utilize the cold inherent in the liquefied natural gas for external duty.

In this specification, the expression liquefied natural gas means a mixture of low molecular weight hydrocarbon gases obtainable from natural deposits of such gases in the ground or associated with crude oil.

It is increasingly the practice to liquefy natural gas for convenient transport to places where it can be utilized. On arrival, the liquefied natural gas is vaporized and used as a fuel or for other purposes, some of its components being more advantageous for some of the purposes than others. At the same time, the considerable expense of liquefying the natural gas in the first instance can be partly recovered by utilizing the liquefied natural gas as a sink of heat, namely by utilizing the cold inherent in the liquefied natural gas and made available for use in the course of their vaporization.

It has now been found that by vaporizing the liquefied natural gas in two stages not only can its components be separated into different fractions having different properties (namely an easily vaporizable fraction hereafter called lean gas and a heavier more slowly vaporizable fraction hereafter called rich gas), but that the sources of cold constituted by the successive stages of vaporization are themselves of diiferent qualities and can be used advantageously for different purposes.

According to the present invention, therefore, a method of vaporizing liquefied natural gas comprises: heating the liquefied natural gas under constant pressure .to give a lean gas and leave a residual liquid, heat for the vaporization being provided by a first heat exchange medium which is-thereby cooled to give a first source of cold for external duty; and heating the residual liquid under constant pressure to give a rich gas, heat for the vaporization being provided by a second heat exchange medium which is thereby cooled to give a second source of cold for external duty.

In practicing the invention, if it is desired to produce the rich gas at a higher pressure than that at which the liquefied natural gas is available, the pressure on the residual liquid from the first stage of vaporization is preferably increased before the residual liquid is completely vaporized. Increasing the pressure on the residual liquid will also provide a convenient way of adjusting the final temperature of the second heat exchange medium, because the greater the pressure on the residual liquid the higher will be the lowest temperature at which the residual liquid will be completely vaporized.

3,253,418 Patented May 31, 1966 The present invention will now be illustrated by the following example, in which reference is made to the accompanying drawing which is a flow diagram for practicing the invention.

Example A mixture of liquefied gases under a pressure of 14.7 p.s.i.a. and at a temperature of 240 F. consisting of methane, 40 mol percent; and propane, 20 mol percent enters the system at a constant rate via a liquid line 1 and passes into the cold side of a heat exchanger 2. In the cold side of the heat exchanger 2, the mixture of liquefied gases is heated to a temperature of 160 F., half of the mixture being vaporized to give a lean gas mixture consisting of methane, 77.8 mol percent; ethane, 21.8 mol percent; and propane, 0.4 mol percent and leave a residual liquid consisting of methane, 2.2 mol percent; ethane, 58.2 'mol percent; and propane, 39.6 mol percent. The lean gas mixture leaves the heat exchanger 2 via a gas line 3. The heat for producing the part vaporization. of the mixture of liquefied gases is provided by gaseous ethylene under a pressure of 17 p.s.i.a condensing at a temperature of l50 F. in the warm side of the heat exchanger 2. Liquid ethylene leaves the heat exchanger via a liquid line 4 to be used as a refrigerant. Fresh gaseous ethylene enters the warm side of the heat exchanger 2 from a gas line 5.

The residual liquid in the cold side of the heat ex changer 2 is pumped into a liquid line 6 by a pump 7, and then passes via the pump 7 into a further liquid line 8 under a pressure of 26 p.s.i.a The residual liquid then passes into the cold side of a heat exchanger 9 in which it'is heated to a temperature of 50 F. and completely vaporized to give a rich gas mixture. The rich gas mixture passes from the heat exchanger 9 into a gas line 10.

The heat for producing the complete vaporization of the residual liquid in the cold side of the heat exchanger 9 is provided by a gaseous propane under a pressure of 16 p.s.i.a. condensing in the warm side of the heat exchanger 9 at a temperature at 40 F. Liquid propane leaves the heat exchanger 9 via a liquid line 11 to be used as a refrigerant. Fresh gaseous propane enters the warm side of the heat exchanger 9 from a gas line 12.

In the above procedure, the pump 7 is not necessary but if it is omitted the temperature to which the residual liquid is heated in the cold side of the heat exchanger 9 is --75 P. not 50 F. In this case, the gaseous propane in the warm side of the heat exchanger 9 still condenses at a temperature of 40 F. but it is also subcooled to a temperature of about F.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined by the appended claims.

I claim:

1. Method of vaporizing liquefied natural gas (LNG) comprising (a) heating the LNG under constant pressure by heat exchange with a first heat exchange medium, at a temperature such as to convert the LNG into two major fractions, one of which is a lean gas and the other a residual liquid gas containing the heavier, more slowly vaporizable constituents of the original LNG,

(b) cooling said first heat exchange medium by said 3 heat exchange to provide a first source of low temperature for external duty,

(0) heating the residual liquid gas from step (a), at constant pressure, by heat exchange with a second heat exchange medium, at a temperature such as to convert said residual liquid gas to a rich gas containing said heavier constituents,

(d) and cooling said second heat exchange medium by the heat exchange of step (c) to provide a second source of low temperature, but at a higher temperature than that produced in step (b), for external duty.

2. A method as claimed in claim 1, comprising increasing the pressure on the residual liquid before it is completely vaporized.

3. A method as claimed inclaim 1, said first heat exchange medium being a gas prior to said heat exchange step and being liquefied during said first heat exchange step.

4. Method as claimed in claim 3, said second heat exchange medium being a gas prior to said second heat exchange step and being liquefied during said second heat exchange step.

5. Method according to claim 4, said first heat exchange medium being ethane.

6. Method according to claim 5, said second heat exchange medium being propane.

7. Method according to claim 6, said LNG being composed of substantial par-ts of methane, ethane and propane; said lean gas being composed of methane and ethane with a negligible percentage of propane: said rich gas being composed of ethane and'propane with a negligible percentage of met-bane.

References Cited by the Examiner UNITED STATES PATENTS 12/1950 Martin et a1. 6252 ROBERT A. OLEARY, Primary Examiner.

LLOYD L. KING, Assistant Examiner. 

1. METHOD OF VAPORIZING LIQUEFIED NATURAL GAS (LNG) COMPRISING (A) HEATING THE LNG UNDER CONSTANT PRESSURE BY HEAT EXCHANGE WITH A FIRST HEAT EXCHANGE MEDIUM, AT A TEMPERATURE SUCH AS TO CONVERT THE LNG INTO TWO MAJOR FRACTIONS, ONE OF WHICH IS A LEAN GAS AND THE OTHER A RESIDUAL LIQUID GAS CONTAINING THE HEAVIER, MORE SLOWLY VAPORIZABLE CONSTITUENTS OF THE ORIGINAL LNG, (B) COOLING SAID FIRST HEAT EXCHANGE MEDIUM BY SAID HEAT EXCHANGE TO PROVIDE A FIRST SOURCE OF LOW TEMPERATURE FOR EXTERNAL DUTY, (C) HEATING THE RESIDUAL LIQUID GAS FROM STEP (A), AT CONSTANT PRESSURE, BY HEAT EXCHANGE WITH A SECOND HEAT EXCHANGE MEDIUM, AT A TEMPERATURE SUCH AS TO CONVERT SAID RESIDUAL LIQUID GAS TO A RICH GAS CONTAINING SAID HEAVIER CONSTITUENTS, (D) AND COOLING SAID SECOND HEAT EXCHANGE MEDIUM BY THE HEAT EXCHANGE OF STEP (C) TO PROVIDE A SECOND SOURCE OF LOW TEMPERATURE, BUT AT A HIGHER TEMPERATURE THAN THAT PRODUCED IN STEP (B), FOR EXTERNAL DUTY. 